Study Says Dredging at Conowingo Would Cost a Lot, Help Little

Nov 13, 2014

The Conowingo Dam taken on December 15, 2012
Credit Chesapeake Bay Program via flickr

A coalition of mostly rural Eastern Shore counties has argued in recent months that dredging the silt trapped behind Conowingo Dam on the Susquehanna River would solve many of the Chesapeake Bay’s problems and save them the cost of an expensive clean-up.

But a state and federal task force that has been studying the problem for three years released a report Thursday that said dredging is more expensive than it’s worth. It would cost between $48 million and $267 million just to dredge a year’s worth of sediment—about 3 million pounds. That wouldn’t make a dent in the mess that’s already there and it wouldn’t make much difference to the bay, either.

“We saw very minor temporary improvements to water quality in Chesapeake Bay from even these large amounts of sediment being removed,” Anna Compton, a biologist with the Army Corps of Engineers, said during a conference call.

She said the task force, comprising scientists from Corps, Maryland’s departments of Natural Resources and the Environment, the EPA’s Chesapeake Bay Program and Nature Conservancy, used multiple computer models to reach its conclusions.

But Jim Simpson, whose recent report for the Maryland Public Policy Institute called for massive dredging behind the dam, scoffed at the findings. He said the pile of sediment is an obvious and glaring problem that “nobody wants to do anything about.”

“So they come up with plans to do something else. So they can say they’re doing something when they’re really doing nothing,” he said.

Simpson, who also writes for the Breitbart Report and other conservative
publications, said the dam’s ability to trap sediment has been lost. Restore that ability by dredging, he said, and the problem would be solved. Downstream communities wouldn’t have to spend millions to upgrade sewage treatment plants, homeowners wouldn’t have to upgrade their septic systems and farmers wouldn’t have to be as careful with nutrients running off their fields.

Yet Compton, from the Army Corps, said it would cost $3 billion to dredge back to mid-1990 levels. And the sediment would just pile up again unless something is done to reduce the flow from upstream.

Governor Elect Larry Hogan, who raised the Conowingo issue frequently during the campaign, told the Baltimore Sun he hadn’t read the report, but called the Corps of Engineers a biased source of information.

The Conowingo dam, built in the 1920s, has kept nearly 200 million tons of
sediment from reaching Chesapeake Bay. But the dam has reached what the
scientists call “dynamic equilibrium.” The muck is piled up so high, the dam no longer can hold it back. What comes down the river flows straight through the sluice gates.

Scientists and others had feared that violent storms, like Tropical Storm Lee in 2011, would scour vast amounts of sediment from the pool behind the dam and hurl it into the bay, choking out aquatic life. But the study found that only a small percentage of the plume of sediment visible after that storm and others came from immediately behind the dam. The rest came from farther upstream.

And the Susquehanna wasn’t the only river with a sediment plume, said Bruce Michael, the Maryland Department of Natural Resources’ representative on the task force. They showed up in all of the bay’s tributaries.

“That sediment actually falls out of the water column,” he said. “And even though there is a short term impact with the sediment as it moves downstream it falls out fairly quickly.”

In addition, he said, the sediment from Conowingo and the other hydropower dams on the lower Susquehanna—Holtwood and Safe Harbor—affects only a small portion of the Upper Bay. The problems in other tributaries and throughout the rest of the bay can be attributed to local conditions.

“So we certainly would expect that most of the local jurisdictions would continue to have implementation plans to see the local responses in their tributaries and streams and rivers,” he said.

Meanwhile, Exelon, which owns the dam, has been granted a one year extension on its operating license and has promised to foot the cost of more extensive monitoring of the sediments and water quality.